This invention relates to an output control system of a combined cycle electric power generating system comprising a combination of a gas turbine, a steam turbine, and an electric generator.
As a combined cycle electric power generating system has been known a single shaft exhaust heat recovery type combined cycle in which a gas turbine, a steam turbine and an electric generator are combined into a single shaft. Such combined cycle electric power generating system is controlled by a load control system as shown in FIG. 1 in which a speed setting signal from a speed setter 1 is subtracted by a speed signal detected by a speed detector 6 and indicative of the number of revolutions of the gas turbine 8, a steam turbine 13 and a generator 10 (since these machines are mounted on a single shaft, or their shafts are coupled together into a single shaft, their number of revolutions are the same), and the difference signal is subjected to a proportional amplification by an operational amplifier 3. The output thereof is applied to a fuel adjusting valve 5 through a servo-amplifier 4 to control the opening of the valve 5 so as to control the flow quantity of the fuel supplied to the burner 7 of the gas turbine 9, thus controlling the output thereof.
The output of the steam turbine 13 is determined by the degree of vacuum of a condensor 14 when a steam adjusting valve 12 is fully open or maintained at a constant degree of opening because the enthalpy of the steam generated by an exhaust heat recovery boiler 11 is determined by the enthalpy of the exhaust gas of the gas turbine 9. The output of the generator 10 is determined by the sum of the outputs of these turbines multiplied by the efficiency of the generator. The output of the generator 10 is supplied to an electric power system and displayed by a load indicator 16 through a load detector 15. The operator of the power plant controls the generator load by manipulating the speed setter 1 while observing the load indicator 16.
For the purpose of automating the control operation, a load setter 17 and a subtractor 18 may be added. Then the load setting signal from the load setter 17 is subtracted with the load signal from the load detector 15 in the subtractor 18 so as to vary the set value of the speed setter 1 according to the difference. Thus, the control is effected such that the difference becomes zero, or the load, i.e. the generator output becomes equal to the set load value. In this manner, the single shaft type combined cycle electric power generating system is controlled so as to always output a desired electric power.
Considering the running efficiency of the electric power generating plant, the efficiency under a partial load is substantially lower than that under or near the full load. For this reason, a partial load should be avoided as far as possible. However, the output of the electric power generating plant is not always equal to the full load, and a partial load running is often necessary. Moreover, when one of the machines becomes faulty, the output becomes zero.